JP6855983B2 - Sulfurous acid-based water treatment agent and water treatment method - Google Patents

Description

The present invention relates to a sulfite-based water treatment agent containing a sulfite, which is used as an anticorrosive agent for boilers.

Conventionally, a technique of adding an oxygen scavenger as a water treatment agent to boiler water supply has been known in order to suppress corrosion occurring on a heat transfer surface such as a boiler water pipe that comes into contact with boiler water. Corrosion of the heat transfer surface of the boiler is mainly caused by dissolved oxygen in the water. Therefore, by adding an oxygen scavenger to the boiler water supply, the amount of dissolved oxygen in the water can be reduced and corrosion can be suppressed. As such an oxygen scavenger, for example, sulfites and the like are used.

Has a deoxidizing effect sulphite excellent, itself generates the Sulfate ion (SO 4 ^_2-) are corrosive factors by being oxidized, there is a problem that the corrosion factor in the heat transfer surface of the boiler .. Therefore, there is known a technique for suppressing corrosion by forming a film on the heat transfer surface on the contact surface side with boiler water by adding silicic acid, silicate, etc. to the water treatment agent in addition to sulfites ( For example, see Patent Document 1 below).

In addition to the above, for the purpose of preventing corrosion of the heat transfer surface of the boiler, a pH adjuster is added to the heat transfer surface of the boiler to adjust the pH of the boiler feed water to the alkaline side where corrosion is less likely to occur. A technique for preventing corrosion is known (see, for example, Patent Document 2 described later).

Japanese Unexamined Patent Publication No. 63-166982 Japanese Unexamined Patent Publication No. 2006-274427

However, as disclosed in the examples of the water treatment agent described in Patent Document 1, when the blending concentration of the silicate with respect to the sulfite salt is relatively low, the heat transfer surface of the boiler is sufficiently corroded. In some cases, it could not be suppressed.

Further, regarding the water treatment agent described in Patent Document 2, when sulfite is blended as an oxygen scavenger, sulfite crystallizes depending on the ion balance in the water treatment agent, and the water treatment agent is stably stored. There was a problem that I couldn't do. That is, the current situation is that the conventional water treatment agent containing sulfite cannot achieve both the preferable corrosion resistance of the boiler water pipe and the storage stability of the water treatment agent.

The present invention has been made in view of the above, and an object of the present invention is to provide a sulfite-based water treatment agent which can impart preferable anticorrosion properties and has excellent storage stability.

The present invention is a sulfite-based water treatment agent added to boiler water supply, which contains sulfite, silicic acid or silicate, and alkali metal hydroxide, and the concentration of sulfite is 10 to 10. It is 25% by mass, and the concentration of silicon dioxide derived from silicic acid or silicate is 1 to 15% by mass, and the ratio of potassium ion concentration ([K ⁺ ]) to sodium ion concentration ([Na ⁺ ]) ( [K ⁺ ] / [Na ⁺ ]) relates to a sulfite-based water treatment agent which is 2/5 to 2/1.

Moreover, the sulfite is preferably sodium sulfite.

Moreover, it is preferable to contain potassium silicate as the silicic acid or silicate.

Further, the hydroxide of the alkali metal is preferably potassium hydroxide.

The present invention also relates to a water treatment method comprising a chemical injection step of adding 0.001 to 0.1 parts by volume of a sulfite-based water treatment agent to 100 parts by volume of boiler water supply.

According to the present invention, it is possible to provide a sulfite-based water treatment agent which can impart preferable anticorrosion properties and has excellent storage stability.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.
The sulfite-based water treatment agent according to the present embodiment is used to suppress corrosion that occurs on the heat transfer surface of the boiler that comes into contact with the boiler water.

The sulfite-based water treatment agent according to the present invention contains a sulfite, silicic acid or silicate, and an alkali metal hydroxide. Further, water, particularly pure water, is preferably used as the solvent.

Sulfites are salts of sulfites that have a sulfite group in the molecule. Sulfites are ionized in aqueous solution to produce sulfite ions. Sulfate ion reacts with dissolved oxygen according to the following formula to become sulfate ion. That is, it has a deoxidizing effect that reduces the amount of dissolved oxygen.

2SO ₃ ^2- + O ₂ → 2SO ₄ ^2-

A water treatment agent containing sulfites is added to the boiler water supply and reacts with dissolved oxygen in the boiler feed water as described above, and also reacts with oxygen in the air during storage of the water treatment agent to produce sulfate ions. There is a possibility that Sulfate ion is a corrosive factor that not only has no deoxidizing effect but also causes corrosion on the heat transfer surface of the boiler. Therefore, it is necessary to prevent such oxidation of sulfite ions during storage of a water treatment agent containing a sulfite.

Examples of the sulfite used in the present embodiment include sodium sulfite (Na ₂ SO ₃ ), _{sodium dissulfite (Na 2} S ₂ O ₅ ), sodium hydrogen sulfite (NaHSO ₄ ), and ammonium sulfite ((NH _{4 ) 2} ). SO ₃ ), ammonium hydrogen sulfite ((NH ₄ ) HSO ₃ ), potassium sulfite (K ₂ SO ₃ ), potassium hydrogen sulfite (KHSO ₃ ) and the like can be mentioned, but it is preferable to use sodium sulfite. Since sodium sulfite is a highly safe compound that is also used as a food additive, the use of sodium sulfite as a water treatment agent ensures safety during handling.

The concentration of sulfite in the water treatment agent is 10 to 25% by mass, preferably 15 to 25% by mass. The required amount of sulfite varies depending on the amount of water treatment agent added to the boiler water supply, the dissolved oxygen concentration in the boiler water supply, etc., but 0.001 to 0.1 parts by volume with respect to 100 parts by volume of untreated boiler water supply. When the water treatment agent is added, generally, if the concentration of sulfites in the water treatment agent is less than 10% by mass, a sufficient deoxidizing effect cannot be obtained. Further, when the concentration of sulfites in the water treatment agent exceeds 25% by mass, crystallization of sulfites during storage of the water treatment agent is likely to occur. In addition, since a large amount of sulfate ion, which is a corrosive factor, is generated, preferable corrosion resistance of the boiler water pipe cannot be obtained.

Silicic acid is a compound containing silicon, oxygen, and hydrogen, and silicate is a salt of silicic acid and an alkali metal. These compounds are composed of silicon dioxide (SiO ₂ ) and are collectively called "silica".
Silica is adsorbed on the contact surface side of the heat transfer surface with the boiler water to form a film, and by covering the heat transfer surface of the boiler water pipe with this film, as a film forming component that imparts preferable corrosion resistance to the boiler water pipe. Function. That is, since the film formed by silica covers the heat transfer surface of the boiler water pipe, the influence of dissolved oxygen contained in the boiler water and corrosive factors such as sulfate ion and chloride ion is suppressed, so that the boiler is suppressed. Preferred corrosion resistance of the water pipe can be obtained. In particular, dissolved oxygen and chloride ions develop local anodes on the heat transfer surface, which can lead to corrosion, but silica, which exists as anions and negatively charged micelles in boiler water, is such an anode. It is easy to be adsorbed on the surface, and it is easy to selectively form an anticorrosive film on the portion. Therefore, as compared with other film-forming anticorrosive agents, silica can impart preferable anticorrosive properties to the boiler water pipe with a relatively small amount of addition.

Examples of the silicic acid used in the present embodiment include silicic acid anhydride (SiO ₂ ), orthosilicic acid (H ₄ SiO ₄ ), metasilicic acid (H ₂ SiO ₃ ), meta-silicic acid (H ₂ Si ₂ O ₅ ) and the like. Examples of the silicate include sodium silicate (Na ₂ O · nSiO ₂ ), potassium silicate (K ₂ O · nSiO ₂ ), and the like, and among them, potassium silicate is preferably used. As described above, in the water treatment agent according to the present embodiment, it is preferable to use sodium sulfite as the sulfite. Therefore, by using potassium silicate as the water treatment agent, potassium silicate becomes a source of potassium ions, which will be described later. It becomes easy to balance the potassium ion and sodium ion of the above. As a result, the crystallization of sulfites can be suppressed, so that a water treatment agent having excellent storage stability can be obtained.

The concentration of silicon dioxide derived from silicic acid or silicate in the water treatment agent is 1 to 15% by mass, preferably 5 to 15% by mass. The required amount of silica varies depending on the amount of water treatment agent added to the boiler water supply, the amount of sulfite added, and the corrosive factors in the boiler water supply, but 0.001 to 0.1 volume per 100 parts by volume of the boiler water supply. When the water treatment agent is added, generally, when the silicon dioxide concentration in the water treatment agent is less than 1% by mass, a sufficient film forming effect cannot be obtained, and preferable corrosion resistance of the boiler water pipe cannot be obtained. .. Further, when the silicon dioxide concentration in the water treatment agent exceeds 15% by mass, silica may precipitate during the storage of the water treatment agent, and preferable storage stability of the water treatment agent cannot be obtained.

The alkali metal hydroxide has a function as a pH adjuster that raises the pH of the boiler feed water so that the heat transfer surface of the boiler water pipe is less likely to be corroded. That is, the pH of the boiler feed water is adjusted to the alkaline side by the pH adjuster, so that corrosion of the boiler water pipe is suppressed. Examples of alkali metal hydroxides used as such pH adjusters include sodium hydroxide and potassium hydroxide, which are strong bases and therefore have a high effect of raising pH, or from the viewpoint of preventing scale generation. It is preferably used.
As described above, in the water treatment agent according to the present embodiment, it is preferable to use sodium sulfite as the sulfite. Therefore, by using potassium hydroxide as the water treatment agent, potassium hydroxide becomes a source of potassium ions and will be described later. It becomes easy to balance the potassium ion and sodium ion of potassium ion and sodium ion. As a result, the crystallization of sulfites can be suppressed, so that a water treatment agent having excellent storage stability can be obtained.

In the sulfite-based water treatment agent according to the present embodiment, the ratio ([K ⁺ ] / [Na ⁺ ]) ^{of the potassium ion concentration ([K +} ]) to the sodium ion concentration ([Na ^{+]) is 2/5.} It is ~ 2/1, preferably 2/5 to 3/2.
Although the principle is not always clear, the present inventors use potassium salts such as potassium silicate and potassium hydroxide to use sodium salts when silicates and pH regulators are mixed with sulfites. It was confirmed that the crystallization of sulfites could be suppressed and good storage stability could be obtained as compared with the case of using it. That is, when the ratio of the potassium ion concentration to the sodium ion concentration is within the above range, it is possible to suppress the precipitation and crystallization of sulfites.

The sulfite-based water treatment agent according to the present embodiment may contain components other than the above-mentioned components as long as the above-mentioned effects are not impaired. For example, catalysts used to improve the deoxidizing capacity of sulfites, other oxygen scavengers such as vitamin C and its salts, tannins, other scale dispersants such as citric acid and polyacrylic acid salts, aminomethyl. Neutralizing amines such as propanol and cyclohexylamine may be contained.

Subsequently, a water treatment method using the water treatment agent according to the present embodiment will be described.
The water treatment method according to the present embodiment includes a chemical injection step of adding 0.001 to 0.1 parts by volume of the water treatment agent according to the present embodiment to 100 parts by volume of boiler water supply.

In the chemical injection step according to the present embodiment, the injection amount of the water treatment agent in the boiler feed water is adjusted to be within the above range, and the water treatment agent is dispensed into the water supply channel that supplies water to the boiler. By setting the injection amount of the water treatment agent in the boiler feed water within the above range, the sulfite ion, silica and the like in the boiler feed water can be set to a preferable concentration. Further, the injection amount of the water treatment agent may be adjusted within the above range according to the dissolved oxygen concentration in the boiler feed water. Since the dissolved oxygen concentration in the boiler feed water is greatly contributed by the water temperature, the injection amount may be adjusted according to the water temperature of the boiler feed water. By adjusting the injection amount according to the dissolved oxygen concentration in the boiler feed water, favorable corrosion resistance of the water treatment agent can be obtained, and unnecessary chemical injection can be suppressed, so that the cost can be reduced. In addition, it is possible to prevent an increase in corrosive factors due to unnecessary chemical injection and scaling of silica, and stable water management can be performed.

As long as the injection amount of the water treatment agent in the boiler water supply is within the above range, the method of chemical injection is not particularly limited, and the water treatment agent may be continuously injected into the boiler water supply or intermittently. Good. In addition, a tank is provided to store the medicated boiler water supply for a certain period of time before it is supplied into the boiler system, and the water treatment agent is injected so that the amount of the water treatment agent injected into the boiler water supply in the tank is within the above range. The amount may be adjusted.

Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

<Examples 1 to 7 and Comparative Examples 1 to 6>
Table 1 shows sodium sulfite (manufactured by Wako Pure Chemical, special grade reagent), potassium silicate (manufactured by Wako Pure Chemical, potassium silicate solution), potassium hydroxide (manufactured by Wako Pure Chemical, special grade reagent) and distilled water as a solvent. Mix so that the content (unit: mass%) shown in Table 1 and the ratio of potassium ion concentration to sodium ion concentration ([K ⁺ ] / [Na ⁺ ]) in the water treatment agent are the values shown in Table 1. Therefore, the water treatment agents of each Example and Comparative Example were prepared.

[Storage stability]
The water treatment agents of Examples and Comparative Examples were stored at −5 ° C. and 5 ° C. for 270 days. The appearance of the water treatment agent after storage was visually observed to confirm the presence or absence of changes. If there is no change, it is set as "A". If crystals or precipitates are settled at -5 ° C, it is set as "B". If there is no change, crystals or precipitates are settled at -5 ° C and 5 ° C. Was evaluated as "C", and the evaluation B or higher was judged to be acceptable, and C was judged to be unacceptable. The results are shown in Table 1.

[Deoxidizing effect]
The water treatment agents of Examples and Comparative Examples were added at 400 mg / L to distilled water at 50 ° C. adjusted to pH 11, and the dissolved oxygen concentration 30 minutes after the addition was measured with a dissolved oxygen concentration meter. The amount of reduced dissolved oxygen before the addition of the water treatment agent is evaluated as "A" when it is less than 1.0 ppm, "B" when it is 1.0 to 2.0 ppm, and "C" when it exceeds 2.0 ppm. Passed and C were judged to be rejected. The results are shown in Table 1.

[Anti-corrosion]
The corrosion resistance of the heat transfer surface of the boiler water pipe was evaluated when the water treatment agents of Examples and Comparative Examples were injected into the boiler water supply. Here, soft water to which 400 mg / L of a water treatment agent is added is supplied to the experimental once-through boiler with an evaporation amount of 1.35 kg / hour for boiler water supply, and blown while continuously generating steam at a pressure of 0.3 MPa. The boiler was operated at a rate of 10%. Corrosion resistance was evaluated by the maximum depth of the pitting corrosion (which indicates pitting corrosion that occurs on the contact surface side of the heat transfer tube with the boiler water toward the opposite side in the thickness direction) after 48 hours of operation. .. Specifically, the maximum value of the pitting corrosion depth of less than 50 μm was evaluated as “A”, 50 to 150 μm was evaluated as “B”, and the maximum value of more than 150 μm was evaluated as “C”. .. The results are shown in Table 1.

From the comparison between Example 2 and Comparative Example 1, it was confirmed that the water treatment agent having a sulfite content of less than 10% by mass did not obtain a preferable deoxidizing effect. Further, from the comparison between Example 3 and Comparative Example 2, it was confirmed that when the amount of sulfites exceeds 25% by mass, the storage stability of the water treatment agent deteriorates and the anticorrosion property also deteriorates.
From these results, it was confirmed that by setting the concentration of sulfites in the water treatment agent to 10 to 25% by mass, it is possible to achieve both a preferable deoxidizing effect and storage stability and corrosion resistance of the water treatment agent.

From the comparison between Example 4 and Comparative Example 3, it was confirmed that the water treatment agent having a silicic acid or silicate-derived silicon dioxide concentration of less than 1% by mass did not obtain preferable anticorrosion properties. Further, from the comparison between Example 5 and Comparative Example 4, it was confirmed that when the concentration of silicon dioxide derived from silicic acid or silicate exceeds 15% by mass, the storage stability of the water treatment agent deteriorates.
From these results, it was confirmed that by setting the concentration of silicon dioxide derived from silicic acid or silicate in the water treatment agent to 1 to 15% by mass, both preferable anticorrosion property and storage stability of the water treatment agent can be achieved.

From the comparison between Example 6 and Comparative Example 5, the ratio of the potassium ion concentration ([K ⁺ ]) to the sodium ion concentration ([Na ⁺ ]) in ^{the water treatment agent ([K +} ] / [Na ⁺ ]] ) Is less than 2/5, it was confirmed that favorable storage stability cannot be obtained. Further, from the comparison between Example 7 and Comparative Example 6, the ratio of the potassium ion concentration ([K ⁺ ]) to the sodium ion concentration ([Na ⁺ ]) in ^{the water treatment agent ([K +} ] / [Na]). ^{It was confirmed that a water treatment agent having a +} ]) ratio of more than 2/1 also did not obtain favorable storage stability.
From these results, the ratio ([K ⁺ ] / [Na ⁺ ]) of the potassium ion concentration ([K ⁺ ]) to the sodium ion concentration ([Na ⁺ ]) in the water treatment agent was 2/5 to 2 /. When it was 1, it was confirmed that preferable storage stability of the water treatment agent could be obtained.

Claims (5)

Sulfurous acid-based water treatment agent added to boiler water supply
Containing sulfites, silicic acid or silicates, and alkali metal hydroxides,
The concentration of sulfite is 10 to 25% by mass,
The concentration of silicon dioxide derived from silicic acid or silicate is 1 to 15% by mass.
A sulfite-based water treatment agent having a ratio ([K ⁺ ] / [Na ⁺ ]) of a potassium ion concentration ([K ⁺ ]) to a sodium ion concentration ([Na ^{+]) of 2/5 to 2/1.} The sulfite-based water treatment agent according to claim 1, wherein the sulfite is sodium sulfite. The sulfite-based water treatment agent according to claim 1 or 2, which contains potassium silicate as the silicic acid or silicate. The sulfite-based water treatment agent according to any one of claims 1 to 3, wherein the alkali metal hydroxide is potassium hydroxide. A water treatment method comprising a chemical injection step of adding 0.001 to 0.1 parts by volume of the sulfite-based water treatment agent according to any one of claims 1 to 4 to 100 parts by volume of boiler water supply.